Lidar observations of the diurnal variations in the depth of urban mixing layer: A case study on the air quality deterioration in Taipei, Taiwan

• Published in: The Science of the total environment Vol. 374; no. 1; pp. 156 - 166
• Main Authors: Chou, Charles C.-K., Lee, C.-T., Chen, W.-N., Chang, S.-Y., Chen, T.-K., Lin, C.-Y., Chen, J.-P.
• Format: Journal Article
• Language: English
• Published: Shannon Elsevier B.V 01.03.2007; Elsevier Science
• Subjects: Aerosols - analysis; Air Pollutants - analysis; Air Pollution; Analysis methods; Applied sciences; Atmospheric pollution; Environmental Monitoring - methods; Exact sciences and technology; LIDAR; Meteorological Concepts; Pollution; Seasons; Taiwan; Tropospheric aerosols; Urban air quality; Urban mixing layer; Taiwan; Taiwan, Taipei; Urban air quality; LIDAR; Urban mixing layer; Tropospheric aerosols; Atmospheric condition; Light scattering; Ground based measurement; Backscattering; Air quality; Urban area; Diurnal variation; Layer thickness; Mixing layer; High atmospheric pollution episode; Laser beam; Atmospheric boundary layer; Aerosols; Air pollution; Mixing ratio; Concentration distribution
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2006.11.049

An aerosol light detection and ranging (LIDAR) system was used to measure the depth of the atmospheric mixing layer over Taipei, Taiwan in the spring of 2005. This paper presents the variations of the mixing height and the mixing ratios of air pollutants during an episode of air quality deterioration (March 7–10, 2005), when Taipei was under an anti-cyclonic outflow of a traveling high-pressure system. It was found that, during those days, the urban mixing height reached its daily maximum of 1.0–1.5 km around noon and declined to 0.3–0.5 km around 18:00 (LST). In terms of hourly averages, the mixing height increased with the ambient temperature linearly by a slope of 166 m/°C in daytime. The consistency between the changes in the mixing height and in the ambient temperature implied that the mixing layer dynamics were dominated by solar thermal forcing. As the cap of the mixing layer descended substantially in the afternoon, reduced dispersion in the shallow mixing layer caused the concentrations of primary air pollutants to increase sharply. Consequently, the pollutant concentration exhibited an anti-correlation with the mixing height. While attentions are usually focused on the pollution problems occurring in a morning inversion layer, the results of this study indicate that the air pollution and its health impacts could be even more severe as the mixing layer is getting shallow in the afternoon.